AU664626B2

AU664626B2 - Doppler compensation in mobile radio

Info

Publication number: AU664626B2
Application number: AU41627/93A
Authority: AU
Inventors: Paul Bune
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1992-07-07
Filing date: 1993-06-30
Publication date: 1995-11-23
Anticipated expiration: 2013-06-30
Also published as: EP0578156A3; AU4162793A; DE4222236A1; EP0578156A2

Description

r 7 1 jV P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT 00*0 00) U *q 0 *00 on* *4 06 4d Invention Title: "DOPPLER COMPENSATION IN MOBILE RADIO" The following statement is a full description of this invention, including the best method of performing it known to us:- I 2 This invention relates to mobile radio systems, and in particular to compensation for Doppler shift in a mobile radio signal propagating between a base station and a mobile station moving past the base station.

A mobile radio signal propagating between a base station and a mobile station moving relative to the base station is subject to the influence of the Doppler effect, which effects a Doppler shift in the frequency range between the transmitted mobile radio signal and the received mobile radio signal.

The Doppler shift is dependent on the speed and direction of the mobile station, which can lead to time changes of the Doppler shift.

To compensate a Doppler shift in a mobile radio signal, methods, such as those described in the two part article "Digital Radio Transmission in the GSM System (GSM: Global System for Mobile Communications)" from "Funkschau" 1991, Magazine 22, pages 68-74 and Magazine 23, pages 77-78, are known.

The method in the above article proceeds from a radio transmission according to the TDMA method (IDMA: Time Division Multiple Access). To establish a radio connection between a base station and a mobile station, the base station transmits a frequency-correction burst, which the mobile station detects to synchronise with the base station frequency. If the mobile station is moving relative to the base station, the above synchronisation is made with a frequency which deviates from the frequency of the base station by a Doppler frequency.

4 After a subsequent time synchronisation with the aid of a synchronisation burst, the radio connection is established. As is further described, the S transmission time of the mobile station is adaptively controlled to compensate the propagation time change of the mobile radio signal while the radio 25 connection is established and the distance between the mobile station and the base station changes constantly. The current propagation time is constantly determined by means of a two way measurement via a special random access burst.

The frequency synchronisation carried out with the frequency correction 30 time slot effects a first, rough compensation of the Doppler shift. The continuous monitoring of the transmission time takes the changing distance between the mobile station and the base station into account and thereby indirectly leads to a further compensation of the Doppler shift.

3 When the mobile station moves past the base station the magnitude and the preceding sign of the Doppler shift change. Such Doppler shift time characteristics, as they occur e.g. with railway radio traffic with a frequency change greater than 1 kHz/sec, require fast, complex algorithms and control circuits when used according to the above, conventional methods.

It is an object of the present invention to provide a method and a circuit arrangement to compensate for Doppler shift in a mobile radio signal, so that the mobile radio connection is guaranteed even when the mobile radio station moves past the base station quickly.

According to the invention there is provided a method of compensating for Doppler shift in a mobile radio signal propagating between a base station and a mobile station moving past the base station, said method comprising the steps of: By means of the mobile radio signal propagating between the base station and the mobile station approaching the base station, predictive values which characterise an expected time characteristic of the Doppler shift, and by means of the predictive values, provide feedforward control such that the Doppler shift is compensated for in accordance with the time characteristic.

According to a further aspect of the invention there is provided a circuit arrangement for compensation for Doppler shift in a mobile radio signal propagating between a base station and a mobile station moving past the base o 25station, wherein an evaluation unit and a function generator are provided, the evaluation unit, by means of the mobile radio signal, determining predictive values which characterise an expected time characteristic of the Doppler shift; and the function generator, by means of the characteristic values, forms a control function signal which controls a transceiver for the mobile radio signal in accordance with the time characteristic so as to compensate for the Doppler 30 shift.

The compensation of the Doppler shift according to the invention is achieved by a feedforward control of a transceiver of the mobile radio signal, which accesses predictive values previously determined from the mobile radio r ._7s 4 signal, the values characterising the expected time characteristic of the Doppler shift.

Re-adjustment time need not be considered and does not limit an application of the invention for compensating fast changing Doppler shifts.

With increasing rate of acceleration of the Doppler shift, the number of values necessary to characterise the time characteristic of the Doppler shift decreases.

For example, only two values (magnitude and time of occurrence) are required to characterise a step-like time characteristic (Doppler step). Time characteristics with a higher order, i.e. with a steady, gliding characteristic, require a larger number of necessary values.

Preferably, a Doppler shift is compensated whose time characteristic corresponds to a Doppler step and which only requires the determination of two values for its characterisation. The characteristic value of the magnitude and the characteristic value of the time of occurrence of the Doppler step are determined by means of the mobile radio signal. For example, in railway radio traffic, the mobile station located in the train driver's cabin transmits the speed VMS and the current track position sMS of the train to the base station. There, the characteristic values of the expected Doppler step are determined from the above values as follows: Magnitude: fVAL 2 VMS fO/c Time of occurrence: TVAL (sMS sBTS) VMS where fO is the carrier frequency of the transmitted mobile radio signal, c is the light speed constant and sBTS is the track position of the base station.

i/ o25 The necessary prerequisites for the above method, such as a nearly constant speed VMS and a straight-line track in close proximity to the base station are given, e.g. with railway radio traffic.

Preferably, the two values of the Doppler step (magnitude and time of occurrence) are derived from a propagation time delay of the mobile radio signal.

This is particularly advantageous with mobile radio signal systems, such as the GSM system, where the propagation time delay of the mobile radio signal between the mobile station and the base station is already determined by a system-inherent routine. In the previously described GSM system, the 1.1 propagation time delay is measured continuously and used to monitor the time of transmission of the mobile station.

The propagation time delay is evaluated according to claim 2, until it falls below a threshold value. This utilises the time to evaluate the propagation time delay, in which the mobile station approaches the base station up to a minimum distance determined by the threshold value.

The value of the threshold value is used to determined the time required to determine predictive values, so that a compensation of the Doppler step by means of the above values is made in the manner of a feedforward control.

From the propagation time delay a series of propagation time delay values is determined and stored, which represents the instantaneous values of the propagation time delay. These can be interrogated directly in the described GSM system.

By interpolation linear regression) of the stored propagation time delay values, a time gradient gradTP of the propagation time delay is determined, from which, and via the speed VMS of the mobile station, a predictive value fVAL is derived as the magnitude of the Doppler step: fVAL 2. VMS .fO c, VMS c grad TP.

20 A further predictive value TVAL, as the time of occurrence of the Doppler step, is determined advantageously by means of extrapolating the stored propagation time delay values. Therefore, the intersection point of the S° propagation time delay values with the time axis determines the required time of occurrence.

S25 Preferably, three predictive values are determined which completely a" o characterise the time characteristic of the Doppler shift as a ramp function.

This method can be used advantageously, e.g. when the base station is positioned at a minimum distance Dmin from the travel path of the mobile station, so that the Doppler shift occurs in the manner of a non-ideal Doppler step with a finite slope. The first predictive value fVAL of the magnitude and the second predictive value TVAL of the time of occurrence can, e.g. as previously described, be determined by the propagation time delay of the mobile radio signal. The slope is characterised by the third predictive value grad fVAL, 2 i 6 which can then be determined as follows: grad fVAL VMS VMS f0/c Dmin c grad TP c grad TP .fO/c Dmin c grad TP grad TP f0/Dmin.

In order that the invention may be readily carried into effect, embodiments thereof will now be described in relation to the accompanying drawings, in which: Figure 1 shows a mobile station approaching a base station, and the time characteristic of a Doppler shift which is to be expected when the mobile station moves past the base station.

Figure 2 shows a mobile station moving very close past a base station, the time characteristic of the propagation time delay of the mobile radio signal, and the derived time characteristic of a Doppler step.

Figure 3 shows a flow diagram of a Doppler-Compensation-Method by means of a propagation-time delay evaluation Figure 4 shows the passing of a mobile station at a distance, the time characteristic of tie propagation time delay and the derived, ramp-shaped, time characteristic of a Doppler shift.

Figure 5 shows a circuit arrangement to execute a Doppler compensation by means of a propagation time delay evaluation.

Figure 1 is designed to show a clear overview of the application of a S° circuit arrangement 200 according to the invention. Real practical examples of a 0o the invention will be described later with the help of further Figure.

Figure 1 shows the situation of a mobile station MS approaching a base *25 station BTS at a time t 0 and Figure 1 shows a time characteristic DS of a Doppler shift in a mobile radio signal RCOM between the mobile station and the base station, which can be expected with the mobile station moving past the base station, and which is designated by a multiple of predictive values xVAL.

These predictive values are determined within the circuit arrangement 200, which is connected to a transceiver TRC of the mobile station, from the mobile radio signal RCOM, and characterise e.g. a magnitude fVAL of the maximum Doppler shift difference, a slope grad fVAL of the greatest Doppler shift change and a time of occurrence TVAL, when this slope occurs.

7 The predictive values, which are determined while the mobile station approaches the base station, determine the expected time characteristic DS of the Doppler shift, so that the circuit arrangement 200 controls the transceiver TRC of the mobile station on time and corresponding to the time characteristic, and compensates the Doppler shift. The compensation can be carried out e.g. by a coordination of an oscillator frequency in a converter stage of the transceiver.

The compensation can be carried out in either the receiver or the transmitter of the mobile station and the base otation, so that the control in both stations is by means of one circuit arrangement. The compensation of the Doppler shift can however also be carried out in one transceiver, i.e. in the transmitter and receiver of one station, so that only one circuit arrangement need be employed.

In the following, concrete practical examples of the invention will be explained: Figs. 2 and 3 show time characteristics, resp. a flow diagram to clarify the compensation of a Doppler step according to the invention.

The mobile station MS shown in Figure 2 moves towards a base station BTS, and a time t 0 and a later time t 3T are shown graphically. The mobile radio signal RCOM, between the mobile station and the base station, propagates with an initially decreasing propagating time delay, which is interrogated at intervals of time T and which shows correspondingly different propagating time delay values TPi.

Within the already mentioned GSM system, the propagation time delay is redetermined within a TDMA time frame, so that the current propagation time delay can be interrogated at intervals of time of approx. T 4.6 ms.. During this interval of time, a mobile station travelling at a great speed of e.g. 300 km/h would only travel a distance of 38 cm. When a greater interva! of time is selected, as for example, T 46 ms, so that a propagation time delay value TPi is interrogated at an interval of 10 TDMA time frames, it is sufficient for the evaluation of the time characteristic of the propagation time delay and for the determination of the predictive values (fVAL, TVAL) of an expected Doppler step

ADS.

By selecting the interval of time T and determining a threshold value TPthres, which determines when the determination of propagation time delay values TPi is terminated, it is possible to limit the number of the determined and r V 8 stored propagation time delay values (here e.g. TPO TP3). A fast evaluation to determine the values can therefore be made, and a remaining interval of time Sbe guaranteed, which is required for the compensation of the Doppler step D characterised by the predictive values, such as magnitude fVAL and time oi occurrence TVAL.

The propagation time delay values TPi are evaluated so that a time gradient gradTP is determined by interpolation, from which the value of the magnitude fVAL is derived as follows, and where fO is the carrier frequency of the mobile radio signal: fVAL 2. f0 gradTP.

Furthermore, the predictive value of time of occurrence TVAL of the expected Doppler step is determined by extrapolation, so that in the propagation time delay graph the point where the extended interpolation line intersects the time axis determines the time of occurrence. If the interpolatioi line intersects the propagation time delay value TPO as shown in the example of Figure 1, the time of occurrence of the Doppler step is determined as folllows: TVAL TPO gradTP.

The time characteristic ADS of the Doppler shift, resulting from the predictive values, is used for the compensating feedforward control of a transceiver, as is done here in a base station.

The practical example explained in the following with the help of Figure 4 proceeds from a mobile station MS moving past a base station BTS, and a minimum distance Dmin between the driving path of the mobile station to the base station.

The Doppler shift time characteristic DS to be expected will be displayed o as a constant differentiable function and will be approximated by a ramp function SDS. To determine the values xVAL which charauterise these ramp functions, propagation time delay values TPi will be determined and evaluated, in the manner already described. Although there is no linear change of the propagation time delay TP, a linear interpolation and a linear extrapolation will be carried out. The ramp function corresponds with a non-ideal Doppler step i with a finite slope grad fVAL.

The values fVAL; TVAL determined from the propagation time delay ji values TPi correspond with, comparative to the previous practical example, the magnitude fVAL or the time of occurrence TVAL of the Doppler step. The now additional value grad fVAL of the slope can be determined as follows: grad fVAL c gradTP gradTP fO/Dmin.

The compensation of the Doppler shift is carried out by means of ti j determined predictive values, in the manner already described.

A practical example of a circuit arrangement according to the invention and Figure 5, to execute a method as sketched in Figure 3, is described in the following.

The circuit arrangement 200 comprises a function generator FGEN and an evaluation unit EVU, which contains a comparator stage COMP, a weighting stage AMP and a central processing unit CU with a memory MEIVI. The evaluating unit EVU is connected with a transceiver TRC for the mobile radio signal RCOM. The function generator FGEN is connected downstream of the evaluation unit. The transceiver TRC is furthermore connected with an already known compensation stage FC.

Both the output of compensation stage FC and the output of function generator FGEN are connected to an adding stage ADD, whose output is connected to a voltage controlled oscillator VCO contained within the transceiver.

The evaluation unit, with the help of decreasing propagation time delay values TPi of the mobile radio signal RCOM, determines two predictive values xVAL; o. these are fVAL, TVAL, as follows: *At intervals of time T, a single propagation time delay value TPi is transferred by the transceiver TRC for the mobile radio signal to the comparator stage COMP. There, this (current) propagation time delay value TPi is compared with a threshold value TPthres. When the current propagation time delay value does not fall below this threshold value TPthres, the propagation time delay value is stored in memory MEM of the central processing unit CU.

In the central processing unit CU, a tirri gradient gradTP of the propagation time delay is interpolated from the stored propagation time delay values TPi. Therefore, the time gradient is available at the end of an interpolation routine INPOL. By a subsequent weighting AMP of this time il 1 7 gradient gradTP, the value fVAL is determined as the magnitude of an expected Doppler step ADS.

The value TVAL as time of occurrence of the Doppler step is determined in the central processing unit CU by an extrapolation EXFPOL of propagation time delay values TPi.

The two above described values fVAL and TVAL are available at the output of the evaluation unit EVU and are then transferred to the function generator FGEN which generates a control function signal DSCOMP from both values. This is a control voltage, whose time characteristic follows the inverted time characteristic ADS of the Doppler step. A compensation of the Doppler shift with its step-like characteristic is achieved in the mobile radio signal by controlling the voltage-controlled oscillator VCO in a converter stage of the transceiver TRC.

In this example the compensation is "only" carried out on the receiver side, so that the circuit arrangement is used in both the demodulation stage of the mobile station and of the base station. The feedforward controlling circuit arrangement 200 is used together with the compensation stage FC which here, for example, executes the already mentioned, conventional frequency synchronisation by means of a frequency correction burst. Both correctives are connected to the control input of the voltage-controlled rscillator VCO via adding stage ADD.

The circuit arrangement 200 is always reset when the mobile station starts to approach the (current) base station, so that the control function signal DSCOMP initially equals zero. The compensation stage FC carries out the frequency synchronisation, so that the voltage- controlled oscillator VCO is IS II S, modulated to an oscillator frequency which corresponds to a desired frequency increased by the Doppler frequency.

From the determined time of occurrence TVAL on, the oscillator frequency is decreased by twice the Doppler frequency (Duppler step magnitude) by means of control function signal DSCOMP. Fast and abrupt change of a Doppler shift are therefore compensated by the circuit arrangement according to the invention and the conventional circuit is supplemented.

i F i 1 _i _il

Claims

1. A method of compensating for Doppler shift in a mobile radio signal propagating between a base station and a mobile station moving past the base station, said method comprising the steps of: By means of the mobile radio signal propagating between the base station and the mobile station approaching the base station, provide predictive values which characterise an expected time characteristic of the Doppler shift, and by means ef the predictive values, provide feedforward control such that the Doppler shift is compensated for in accordance with the time characteristic.

2. A method as claimed in claim 1, wherein two predictive values are determined which characterise the time characteristic of the Doppler shift as a Doppler step, with the first characteristic value characterising the magnitude of the Doppler step, and the second characteristic value characterising the time of occurrence of the Doppler step.

3. A method as claimed in claim 2, wherein the two predictive values are determined from the deureasing propagation time delay of the mob !e radio signal as follows: At intervals of time, propagation time delay values are determined and stored until such a value falls below a threshold value; by interpolation of the stored propagation time delay values, a time gradient of the propagation time delay is determined from which the first predictive value is derived as the magnitude of the Doppler 25 step, and S by extrapolation of the stored propagation time delay values, the second predictive value is determined as the time of occurrence of the Doppler step such that at the time of occurrence, the propagation time delay is a minimum.

4, A method as claimed in claim 1, wherein three predictive values are R determined which characterise the time characteristics of the Doppler shift as a 1 n*Yramp function, with the first predictive value characterising a maximum amount 12 by which the Doppler shift changes, the second predictive value characterising a time when the Doppler shift becomes zero, and the third predictive value characterising a slope of the ramp function.

A circuit arrangement for compensation for Doppler shift in a mobile radio signal propagating between a base station and a mobile station moving past the base station, wherein an evaluation unit and a function generator are provided, the evaluation unit, by means of the mobile radio signal, determining predictive values which characterise an expected time characteristic of the Doppler shift; and the function generator, by means of the characteristic values, forms a forward control function signal which controls a transceiver for the mobile radio signal in accordance with the time characteristic so as to compensate for the Doppler shift.

6. A circuit arrangement as claimed in claim 5, wherein the evaluation unit includes a comparator stage, a weighting stage, and a central processing unit including a memory by means of which two predictive values which I characterise the time characteristic of the Doppler shift as a Doppler step are determined, the comparator stage reading a propagation time delay value of the mobile radio signal from the transceiver at intervals of time, and passes it on to the memory if the respective propagation time delay value is not less than a threshold value; the stored propagation time delay values are transferred from the memory into the central processing unit, where they are subjected to an interpolation and an extrapolation; by means of the interpolation, the central processing unit determines a time gradient of the stored propagation time delay S values, which, after being weighted by the weighting stage, determines a first S- 25 characteristic value as a magnitude of the Doppler step, and by means of the extrapolation, a second characteristic value is determined as a time of occurrence of the Doppler step.

7. A method and circuit arrangement substantially as herein described with reference to Figures 1 5 of the accompanying drawings. ~RA( DATED THIS TWENTY-FIRST DAY OF SEPTEMBER 1995 o U ALCATEL N. V. (U ABSTRACT A mobile radio signal propagating between a base station and a mobile station moving relative to the base station is subject to the influence of the Doppler effect. The Doppler shift can show an abruptly changing time characteristic which occurs e.g. during railway radio traffic when the mobile station moves past the base station. Known compensation methods for direct and indirect frequency modulation, by means of evaluation of the frequency correction signal or a continuous transmission time control, cannot follow a fast change of Doppler shift lkHz/sec). The invention provides a method and a circuit arrangement (200) for Doppler shift comrensation in a mobile radio signal (RCOM) propagating between a base station (BTS) and a mobile station (MS) moving past the base station. This is achieved by evaluating the mobile radio signal (RCOM) to determine predictive values (xVAL) of the expected time characteristic (ADS) of the Doppler shift and by carrying out a compensating feedforward control of a transceiver (TRC) of the mobile radio signal (RCOM) from this time characteristic. FIGURE 3. t II